Problem: Which of these numbers is prime? ${9,\ 37,\ 38,\ 65,\ 81}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 9 are 1, 3, and 9. The factors of 37 are 1 and 37. The factors of 38 are 1, 2, 19, and 38. The factors of 65 are 1, 5, 13, and 65. The factors of 81 are 1, 3, 9, 27, and 81. Thus, 37 is a prime number.